Process for controlling the operation of a print head

ABSTRACT

A process for controlling the operation of a printing head detects defects in the transport of the printing head along a printed line and if necessary corrects them. A control unit (24) controls a current source (16) which supplies current to a transport motor (12) which moves the printing head (10). The printing elements (14) of the printing head (10) are supplied with pulsed drive current (ID) by a driver component (18). A position indicator (20) records the distance travelled by the printing head (10) along a printed line. According to the invention, the transport time between two printing positions is monitored to detect whether a limit value (G) is exceeded. If the limit value (G) is exceeded, the drive of the printing head (10) is stopped and if necessary the printing head is moved in the opposite direction and/or at least one printing element (14, 15) is operated with less energy that that required for printing. As a result of these measures, a defect due to adhesion of a printing element (14) to a printing ribbon can be detected and eliminated automatically.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention concerns a process for controlling the operation of aprint head that is moved from the printing position in a given directionand within a given transport time to another position after printing acharacter by actuation of at least one printing element, whereby thetransport time is monitored to determine whether a limit value isexceeded, and if the limit value is exceeded, the drive of the printhead is interrupted.

2. Background Art

Processes of this type are used in printers for printing text, data orgraphic information. They are used to bring a print head into a givenprinting position and then produce an impression on a substrate such aspaper by actuation of at least one printing element. To do so, an inkribbon is arranged between the print head and the substrate and comes incontact with the substrate when the printing element is actuated. With anumber of printers, the information to be printed is printed characterby character, i.e., the print head is shifted in the direction of theline and in doing so executes the printing operation.

After printing a line, the printed substrate is advanced by the distanceequal to one line by means of a roller, for example, and then theline-by-line printing operation is carried out again. During printing,the instantaneous position of the print head in a line is detected andcan be relayed by means of a position-reporting signal to a controllerthat actuates a printing element to print a character on reaching acertain position. This principle can be used in an interval-controlledoperation, whereby the movement of the print head is interrupted andthen the printing element is actuated, and it can also be used inso-called "flying" printing. In the latter case, the print head is notstationary when printing a character but instead the printing element isactuated without interrupting the movement of the print head along theline.

In operation of a print head, the print head may adhere to the printingmedium, i.e., the ink ribbon or the substrate, after actuating aprinting element. For example, with a needle print head, whose printingelements are needles, a needle actuated for printing can puncture theink ribbon and hook into it so that it does not return to its restingposition. When the print head is advanced to the next printing position,the needle can be either bent or broken off due to the relative motionbetween the needle and the ink ribbon. This causes destruction of theentire print head. This risk is increased with recent needle printersdue to the fact that the number of needles per print head is high andthin needles that can more easily puncture the ink ribbon are used. Evenwith printing processes that do not use an ink ribbon but use so-calledactive paper as the substrate, it is still possible for a printingelement to penetrate into the substrate and adhere to it by hooking intoit.

In other printing processes, it is also possible for the print head andink ribbon to stick together. For example, in heat transfer printing,where the printing elements are electric heating resistors that transferthe character to the printing stock with the help of a melting inkribbon, there is the danger that the printing element of the print headmight stick to the melting tool adhering to the ink ribbon. The inkribbon, which is made of a plastic film, may also adhere to the printingelements due to overheating or because of surface defects, which thuscan cause interference in operation of the print head.

In addition, interference can also occur in transport of the print headdue to an incorrectly inserted printing stock, due to a paper jam or dueto trouble in the drive system. In all these cases, it would bedesirable to be able to detect these problems promptly in order toprevent damage to sensitive components.

SUMMARY OF THE INVENTION

Therefore, the goal of this invention is to develop a process forcontrolling operation of a print head whereby transport problems will bedetected and eliminated if possible.

This problem is solved for a process of the aforementioned type by thefact that in order to release sticking between the printing element anda printing medium, the print head is moved in the direction opposite thegiven direction whenever the limit value is exceeded and/or at least oneprinting element is operated with reduced power in comparison with thepower applied for printing.

This invention makes use of the finding that due to the sticking of theprint head and the printing medium, i.e., the ink ribbon or the printingstock, the forward movement of the print head is prevented by the inkribbon or the printing stock and it does not reach the next printingposition or reaches it with a delay. By monitoring the transport time,it is possible to detect trouble in transport of the print head andinterrupt the drive of the print head. This prevents damage to the printhead and measures can be initiated to eliminate the transport problem.It is advantageous to use the position signal of the printer, which is aknown feature, for achieving the specified printing positions.

This invention provides for the print head to be moved in the directionopposite the given direction whenever the limit value is exceeded. Thissimple measure makes it possible to release any sticking of the printhead and printing medium. For example, when a needle punctures the inkribbon in a needle print head and the ink ribbon is thus stretched whenthe print head continues to move, the elastic tension between the needleand ink ribbon is reduced due to the reverse movement of the print headto the extent that the needle resiliency which retracts the needle intoa resting position in the print head is sufficient to release it fromthe ink ribbon.

Another possibility of eliminating the sticking of the print head andink ribbon consists of actuating one or more printing elements, wherebythe energy required for this is reduced in comparison with the energyused to print a character. By actuating the printing element, the resultachieved is that energy that results in release of the two parts issupplied at the location where the ink ribbon and print head aresticking together. For example, with a heat transfer printer, theadhesive bond between the print head and the melting ink ribbon issoftened as heat is applied and thus is released. With a needle printer,mechanical energy is applied at the location where the needle is caughtin the ink ribbon by actuating one or more printing elements. Thetensile and compressive forces which thus occur at the point of contacttogether with the resiliency of the needle cause the needle to bereleased from the ink ribbon and returned to its resting position.

Preferably a combination of he aforementioned measures can also be used.This can occur by actuating the printing element during or after thereverse movement of the print head. The two work together in such a waythat adherence of the print head and the ink ribbon can be eliminatedwith a high degree of reliability.

In a process according to this invention, the limit value for which thetime between printing a character and output of the position reportingsignal is monitored is preferably set according to the type of printhead or printing parameters. Such printing parameters include, forexample, the speed at which the printing head moves, the type of inkribbon or the energy that must be expended to actuate the printingelement. As a result of these measures, it is possible to use theprinciple according to this invention for a large number of types ofprint heads and for different printers with several modes of operation.

In a practical refinement of this invention, the reduced energy neededto actuate the printing element is such that no character is printed. Asa result, the measures for eliminating the trouble due to the stickingof the ink ribbon and the print head do not leave any traces on theprinting stock. The quality of the printed result is thus not affectedby these measures.

An advantageous refinement of this invention is also possible due to thefact that one or more printing elements are actuated with reduced energyrepeatedly in succession. When this measure is used with needleprinters, for example, it leads to a vibrating motion between the needleand the ink ribbon. This type of motion facilitates the release of aneedle caught in the ink ribbon.

Another advantageous refinement of this invention may also beimplemented in such a way that the printing elements of the print headare actuated simultaneously with reduced power. What this means for aneedle print head is that all needles are deflected simultaneously andpress against the ink ribbon. In this way the ink ribbon is shifted inthe direction of the printing stock and is stripped away from the needlecaught in it.

Preferably with a needle print head, the energy required to operate theprint element is such that the needle stroke is reduced approximately byone-half. In practice, it has been found that with this stroking motionof the needle or several needles, an optimum result is achieved inreleasing the needles sticking to the ink ribbon on the one hand and onthe other hand there is great security in the fact that no characterwill be printed on the printing stock.

Another refinement of this invention is characterized in that atransport motor provided for moving the print head is short-circuited oris driven with a countercurrent when the limit value is exceeded. Thismeasure causes the motor to be electrically decelerated and the movementof the print head stops after a very short distance. In doing so, theink ribbon and the printing element of the print head are not subjectedto a stress that exceeds their limit of elasticity, i.e., they are notdamaged. Furthermore, the total time needed to eliminate the problem isshortened by this measure.

The process according to this invention can also be carried out in sucha way that a transport motor that is provided for moving the print headis driven with a predetermined number of current pulses to move it inthe opposite direction. These cause a jerky stepwise movement of theprinting head so sudden tensile and compressive forces occur at thepoint of contact between the print head and the ink ribbon. These forcesfacilitate the release of the adhesive bond between the print head andthe ink ribbon. The number of current pulses is adjusted according tothe type of transport motor, the printer, the ink ribbon and otherpressure-specific parameters.

Another practical implementation of this invention provides for theprint head to be positioned after a predetermined period of time on theprinting position which it held before the limit value was exceeded andthen printing is continued in this position. The predetermined time isset according to the type of print head, the transport motor and otherparameters. This design assures almost interruption-free operation ofthe printer. However, if such trouble recurs within a certain period oftime, an error report can be generated when the limit value is exceededa predetermined number of times in another variant of this invention.This makes it possible to indicate trouble that cannot be eliminated bythe process according to this invention so that other measures such asrepair or replacement of the print head can be taken.

In another expedient version of this invention, such a measure mayconsist of the fact that operation of the print head is stopped when thelimit value is exceeded several times within a given period of time.

In an advantageous refinement of the process according to thisinvention, the direction of movement of the print head is determinedwhenever there is an interruption in operation because the limit valuehas been exceeded, and if there is a failure in reversal of directions,another error report is generated. As a result of this measure, errorsthat are manifested by exceeding the limit value and cannot beattributed to sticking of a printing element can be detected reliably.When a printing element sticks to the ink ribbon or the printing stock,an elastic tension occurs and results in a resilient force and thuscauses a reversal in the direction of the print head. For example, aftera print needle has been hooked in the ink ribbon, a needle print headstretches the ink ribbon by a certain amount due to the energy ofmovement of the print head so the resulting resilient force causes theprint head to swing backwards. This reversal in direction can bedetected by means of the position-reporting signal, for example. If thisreversal of direction does not occur, then there is trouble that mightbe caused, for example, by mechanical jamming of the print head or by apaper jam. Consequently, an error report can be generated or operationof the print head can be interrupted.

One practical example of this invention is illustrated below withreference to the figures which show the following.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the basic design of a controller in a block diagram foroperation of a print head.

FIG. 2 shows the signal plots of the driving current for actuation of aprint element as well as the position reporting signal as a function oftime.

FIG. 3 shows a logic flow chart of process steps.

FIG. 4 shows conditions and signal plots in normal operation and whenthere is trouble.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows various functional units of a controller in a printer foroperation of a needle print head 10 which is moved back and forth alonga line of print by a transport motor 12. Transport motor 12 is suppliedwith motor current IM from a power source 16. Needle print head 12 hasas its printing elements needles 14 that are arranged in the form ofcolumns or a matrix in print head 10 and the number of needles will be7, 9, 12 or more depending on the type of print head. A needle 15 isshown separately and will be discussed in detail later in describing atrouble incident. Needles 14 are actuated in printing by anelectromagnetic actuating mechanism, whereby they are deflected fromtheir resting position in print head 10 and are pressed against an inkribbon that transfers a character to printing stock. Theelectromechanical actuating elements (not shown in FIG. 1) are suppliedwith a driving current ID in the form of pulses from a driver component18, where the electric energy can be varied by varying the period andamplitude of the current pulses.

The path covered by print head 10 along a line of print is detected by aposition indicator 20 which may be a conventional path coder thatgenerates two signals that are shifted in phase by 90° at givenintervals. On the basis of these signals, the path covered by print head10 can be determined by counting the coding pulses and its direction ofmovement can be determined by comparison of the phase position of thesignals. Position indicator 20 thus sends a position reporting signalPMS to control unit 24. This signal contains information regarding theinstantaneous position of print head 10 as well as its direction ofmovement.

To perform the printing operation, the control unit 24 receivesadditional control information by way of data line 26. Such informationmay include, for example, the character to be printed, the printingspeed, the start and end of a printing process. Using all thisinformation, control unit 24 generates control signals that are thensent to the power source 16 and to the driver component 18 so themovement of transport motor 12 and the printing of characters arecontrolled. Control unit 24 contains a microcomputer for carrying outits various functions. This microcomputer is equipped with a programmemory and data memory as well as a clock unit. This type of control ofa needle print head is essentially known and therefore the details neednot be explained.

To carry out the process according to this invention, control unit 24has access to other control modules 30 to 36 that are contained assoftware programs in a read-only memory 22 (ROM). These control modulesinclude a monitoring module 30 that monitors the time between actuationof a print element 14 by a driver current pulse ID and the occurrence ofa position reporting signal PMS to determine whether they exceed limitvalue G. Control module 32 controls the direction of rotation oftransport motor 12 in the event of trouble. Control module 34 isprovided in order to generate a given number m of motor current pulsesthat are delivered to transport motor 12 which then converts them tostepping movements. Finally, control module 36 generates a given numbern of pulses that act indirectly by way of driver module 18 in order toactuate printing elements 14. Furthermore, control unit 24 has a troubledisplay 28 which is triggered in the event of trouble and providesnotice of any errors in operation.

FIG. 2 shows signal plots of the position-reporting signal PMS as afunction of the driver current ID. The signal states are labeled aslogic 1 and logic 0 and indicate current flow and a state of rest,respectively. The signal plots differ in normal operation A and in thetrouble case B. In normal operation A, print head 10 is advanced along aline and one or more needles 14 are actuated in so-called "flying"printing. Printing processes are described below on the basis of needles15 which are illustrated separately in FIG. 1. The ink ribbon isstretched along a line with the type of printer described here and isnot moved back and forth with the print head. With the rising edge 40 indriver current ID, needle 15 is deflected in the direction of the inkribbon with a slight delay which is due to the inertia of theelectromechanical actuating element. The ink ribbon, which is made of afabric-like material, is pressed against the printing stock by the tipof needle 15, so it forms a dot-shaped character on the printing stock.When several needles are actuated in this way, a columnar or mosaic-likeprint character is formed on the printing stock. After actuating needle15, it is normally retracted by its resilient force into its restingposition in needle print head 10.

With the descending edge 42 of the driver current, the monitoring module30 becomes active and monitors the time T until occurrence of the nextposition-reporting signal PMS when the given limit value G is exceeded.This limit value is set according to print-specific parameters such asthe type of print head and the printing speed. In normal operation A,the time T is smaller than limit value G, which means that the printingoperation proceeds normally.

In case B, printing operation is interrupted, i.e., print head 10 doesnot reach the next position signaled by a square wave pulse ofposition-reporting signal PMS within the specified period of time afterthe drop in driver current ID. For example, a paper jam or improperlyinserted paper may prevent the forward motion of print head 10. Ingeneral, such trouble can be eliminated only by an operating person.Another problem that often occurs is due to the fact that one of needles14, e.g., needle 15, punctures the fabric of the ink ribbon whenexecuting its back-and-forth motion and thus sticks in the ink ribbon.The ink ribbon thus held up then inhibits the motion of print head 10 sotime T to reach the next position exceeds limit value G. Such a problemis thus reliably detected by the time monitoring function. This problemis automatically eliminated by the process steps according to thisinvention as explained below.

The control functions of control unit 24 that take place in normaloperation and in the event of trouble are illustrated in FIG. 3 in theform of a flow chart. In a first step 50, as described above, print head10 is moved along the line by the transport motor 12 and itsinstantaneous position is determined on the basis of theposition-reporting signal PMS by counting pulse edges. To print acharacter in a given position (step 52), the needles forming thecharacter are selected from printing elements 14 and the correspondingelectromechanical actuating elements are driven with driver current ID.In the next step 54 on the descending edge of the driver current ID, atime monitoring function is active and determines whether the pulse edgeof the position-reporting signal PMS occurs within the limit value timeG (step 56). The time monitoring function is accomplished with the helpof a counting module that is loaded with an initial count that isproportional to limit value G. When starting the time monitoringfunction, the count is decremented according to the cycle of a clocksignal provided by the clock unit of the microcomputer. If aposition-reporting signal PMS occurs before reaching count 0, then thisis a normal printing operation and step 50 is initiated in order tocontinue the printing process. If so much time elapses until occurrenceof the position-reporting signal PMS that the count 0 is reached thenthe next step is step 58, where an inquiry is made as to multipleproblems, the importance of which will be explained below. If there areno multiple problems, then transport motor 12 is stopped in the nextstep 59 in order to interrupt the movement of print head 10. This isdone by short-circuiting the motor winding of the transport motor 12 soa great delay is initiated by the eddy current effect. It would also beconceivable to drive the motor winding with a polarity opposite that ofthe current so the delay would be further increased. A long delay isdesirable in order to compensate as much as possible for the prevailingenergy of motion of print head 10 by transport motor 12 so the danger oftearing the ink ribbon or of bending of needles 15 is reduced.

In the next step 60, the braking effect is stopped. The ink ribbon,which is under elastic tension due to the printing needle being hookedin it, can then be released and then entrains the print head in thedirection of stress relief, which is reported by a position indicator 20as a reversal in the direction of motion. In step 61, the reversal ofdirections is analyzed. If the reversal of directions does not occur,then there is a problem such as a break in the drive connection betweenthe motor and the print head or a paper jam. Then step 70 can beimplemented directly, whereby an error report is generated. When thereis a reversal of directions, the rotational direction of the motor isreversed in step 62, i.e., the direction of motion of print head 10 isthen opposite the direction before the problem. Next the motor windingis driven with a given number m of current pulses IM (step 64), so printhead 10 executes jerky stepping motions that cause or facilitate therelease of needle 15 from the ink ribbon. The number m of current pulsesIM as well as their amplitude and period are set so that print head 10will return approximately to the position it held before the problem. Asthe next measure 66 all needles 14 of print head 10 are actuated. To doso, n current pulses of a defined width are generated by driver module18 as the driver current ID and sent to the electromechanical actuatingelements of needles 14. The energy supplied to the actuating elements isadjusted by varying the pulse period in such a way that needles 14execute approximately half of their back-and-forth motion. With thismovement, needles 14 are pressed against the ink ribbon and strip itaway from the needle 15 stuck in it. Due to this reduced back-and-forthmotion, printing of a character on the printing stock is prevented. Inaddition, a vibrating motion that facilitates the release of needles 15from the ink ribbon is induced by the repeated actuation of needles 14.Actuation of needles 14 can be carried out before or during the brakingof transport motor 12 and also during the jerky reverse movement ofprint head 10. If the printing elements 14 are not actuated until afterthe reverse movement of print head 10, this has the advantage that theink ribbon is released of tension so that no transverse forces areexerted on the needle 15 caught in the ink ribbon.

In the subsequent step 68, the direction of rotation of transport motor12 is reversed again so the direction of movement of print head 10 isthe same as its direction before the interruption. After a waiting time,step 50 is initiated in order to automatically continue the printing atthe point of interruption.

As explained above, an inquiry as to multiple problems is made in step58. This is done in order to detect multiple occurrence of aninterruption or a delay in motion of the print head. Such successiveproblems can be caused, for example, by a paper jam, a defective inkribbon or a needle that is hooked very strongly into the ink ribbon. Ifsuch problems occur repeatedly within a given period of time, then step70 is initiated after a given number of attempts to release the needleand an error report is generated and displayed. The operator of theprinter can then perform the maintenance or repair of the printer. Ofcourse such an error report can also be generated as soon as a singleproblem occurs, in which case it is then possible to perform astatistical analysis of the errors. After display of the multipletrouble signal, operation of the print head is interrupted in thepresent example (step 72).

FIG. 4 illustrates various states in operation of print head 10 as afunction of time t. The plots of the direction of movement of print head10 (print head right, print head left), of the position-reporting signalPMS, of the driver current ID for actuation of print elements 14 and ofthe trouble state S are shown as a function of time. At time t=0, printhead 10 moves from left to right in the direction of the line. The pathcovered by it is signaled by the position-reporting signal PMS in theform of square wave pulses. A driver current ID is flowing which meansthat printing element 15 is being actuated and a character is beingprinted. On the descending edge 80 of driver current ID, the timemonitoring function is actuated and the time T until occurrence of therising edge of the pulse of the position-reporting signal PMS ismeasured. If this time is smaller than limit value G, then no problem Sis indicated whereupon the time monitoring function is reset to theinitial state and started again with the next descending edge 82 ofdriver current ID. In the case that time T reaches limit value G, aproblem S is indicated. Then the measures for releasing a needle thatmight be stuck in the ink ribbon are initiated, i.e., transport motor 12is stopped and moved step by step toward the left in the oppositedirection. Then all needles 14 are driven with a fixed number of drivercurrent pulses ID, where the pulse width b is reduced in comparison withthat in normal operation of a printing element. After multiple operationof printing elements 14, the problem S will generally have beeneliminated and print head 10 is moved back into its original directionand printing is continued.

We claim:
 1. A process for controlling the operation of a print head,said print head having means for driving said print head and a pluralityof printing elements for printing characters at one of a plurality ofprinting positions on a printing medium and within a transport timehaving a predetermined limit value, comprising the steps of:(A) printingat least one said character at a first one of said printing positions onsaid printing medium by actuation of at least one of said printingelements; (B) advancing said print head, using said driving means, in apredetermined direction from said first printing position to a secondone of said printing position; (C) interrupting said driving means ifthe transport time for the advancement of said print head in step (B)exceeds said limit value due to said printing element sticking to saidprinting medium; and (D) releasing said print head element from saidprinting medium by moving said print head in a direction opposite to thedirection that said print head is advanced in step (B).
 2. The processaccording to claim 1, wherein said limit value is set according toprinting parameters of the print head.
 3. The process according to claim2, wherein said reduced energy has a value such that no character isprinted on actuation of said printing element.
 4. The process accordingto claim 2, wherein said printing elements of said print head areactuated simultaneously with said reduced energy.
 5. The processaccording to claim 2, wherein said printing elements of said print headare actuated simultaneously with said reduced energy.
 6. The processaccording to claim 2, wherein said printing element is of the needletype and the reduced energy for actuation of said printing element issuch that the stroke of said printing element is reduced by aboutone-half.
 7. The process according to claim 2, wherein said drivingmeans includes a transport motor and step (C) is performed by:stoppingsaid transport motor when said limit value is exceeded.
 8. The processaccording to claim 7 wherein stopping said transport motor is performedby short-circuiting said transport motor.
 9. The process according toclaim 7, wherein stopping said transport motor is performed by drivingsaid transport motor with a countercurrent.
 10. The process according toclaim 2 wherein said driving means includes a transport motor providedfor moving said print head in said opposite direction and wherein step(D) is performed by driving said transport motor with a predeterminednumber of current pulses.
 11. The process according to claim 2, furthercomprising the step of generating a first error report when said limitvalue is exceeded a preselected number of times.
 12. The processaccording to claim 2, further comprising the step of:(E) switching ofthe operation of said print head within a predetermined period of timewhen said limit value is exceeded a preselected number of times.
 13. Theprocess according to claim 2, wherein step (B) is performed after step(C), and step (C) includes the substeps of:determining said direction ofmovement of said print head; and generating a second error report ifthere is no reversal of direction.
 14. The process according to claim13, further comprising the step of:interrupting the operation of saidprint head when there is no reversal of direction.
 15. A process forcontrolling the operation of a print head, said print head having meansfor driving said print head and a plurality of printing elements forprinting characters at each of a plurality of printing positions on aprinting medium and within a transport time having a predetermined limitvalue, comprising the steps of:(A) printing at least one of saidcharacters at a first said printing position on said printing medium byactuating at least one of said printing elements; (B) advancing saidprint head, using said driving means, in a predetermined direction fromsaid first printing position to a second one of said printing position;(C) interrupting said driving means if the transport time for theadvancement of said print head in step (B) exceeds said limit value dueto said printing element sticking to said printing medium; and (D)releasing said print head element from said printing medium by actuatingsaid printing element with reduced energy.
 16. The process according toclaim 15, wherein step (D) is performed by repositioning said print headafter said limit value is exceeded, at said first printing positionwhere printing is continued.